The present invention relates to a surface inspection system used in inspection process in manufacture of a semiconductor and for inspecting whether foreign objects such as particles spread on a wafer surface are present or not.
A surface inspection system for a wafer is designed so that a laser beam is projected to a surface of the wafer under high-speed rotation, and the laser beam scans the entire surface of the wafer while a laser beam projecting position is moved at a predetermined pitch or at a predetermined rate in a radial direction, and a light reflected from the surface of the wafer is detected. The reflected light is scattered by foreign objects on the surface of the wafer. The scattering condition differs according to the particle size of the foreign object. From the change in the conditions of the detected scattered light, it is possible to identify whether there is foreign object or not and the size of foreign object.
Sensitivity and Responsiveness of a photodetection sensor, which detects the scattered light, can be adjusted by voltage applied on the photodetection sensor. That is, by maintaining the voltage applied on the photodetection sensor at an adequate level, it is possible to detect foreign objects in stable manner. For this reason, when the surface inspection of the wafer is performed, the responsiveness of the photodetection sensor is adjusted (calibrated) by using a calibration wafer, which has the same characteristics as the wafer to be inspected and which is coated with standard particles with a predetermined diameter and in a predetermined number of particles.
In the past, the calibration of the photodetection sensor has been performed as follows: Prior to starting of the inspection, a calibration wafer is brought into the surface inspection system from outside of the surface inspection system at each inspection, and the calibration wafer is loaded on a wafer chuck in the surface inspection system.
A laser beam is projected to the calibration wafer, and the scattered light is received by the photodetection sensor. Then, the voltage applied on the photodetection sensor is adjusted in such manner that spreading condition of foreign objects detectable under photodetecting condition matches with data already known of the calibration wafer.
In the conventional type surface inspection system described above, the calibration wafer is brought in or out manually by an operator, and this results in low working efficiency. Also, the calibration wafer acts as a reference, and it must not be contaminated with particles spread on it. However, because the manual work is involved, there is high possibility that particles are spread on the wafer surface. Because the calibration wafer is brought in from outside or brought out to outside, the surface inspection of the wafer cannot be carried out immediately after the adjustment of sensitivity of the photodetection sensor, and it is unavoidable that a certain time elapses before the surface inspection is performed. For this reason, the changes over time such as drift may occur on the responsiveness of the photodetection sensor, and this may exert influence on detection error.
Further, the calibration wafer must be kept under clean atmosphere outside the surface inspection system. This means that an additional storage space for the calibration wafer or an equipment similar to such storage system is required.